This invention relates to methods and apparatus for detecting the presence and location of a conductor which extends on opposite sides of an air/water boundary, for example, in the case of a command control wire leading from a location on a bank of a body of water to a mine situated in the water. The invention is, of course, applicable to the location of other conductors such as pipes, communication cables, and the like.
An effective sweeping or countermeasure approach to wire controlled mines, in a river for example, requires first the ability to locate the command control wire so that one or more subsequent steps may be taken, such as neutralizing the enemy command station, cutting the command wire to disable the mine, or applying a suitable electrical signal to the wire to cause the mine to explode at a time when no damage will ensue.
The ability to locate such a conductor from a moving vehicle, such as a boat, helicopter, or the like, has been limited to techniques which are either insufficiently reliable or require more complex and expensive equipment than is desirable. For example, one technique has involved towing gradiometer means in the form of first and second, spaced, radio frequency detectors, with the expectation that electromagnetic energy of radio frequency from a radio broadcasting transmitter would be carried by the wire from its air domain to the water domain, where electromagnetic radiations from the wire would there be received by the towed gradiometer means as it passes close by. That technique is described in my patent application Ser. No. 126,938, filed Mar. 22, 1971, and entitled "Wire Detector". While a towed two receiver, radio frequency gradiometer will, in fact, receive such energy, which can be transmitted by cable to the towing vessel for display and/or classification, the requirements of having two radio frequency detectors or receivers housed in a watertight casing and of transmitting data therefrom, either by wire or other telemetering equipment, results in expensive, bulky, tow equipment which is difficult to maintain.
Another approach has been to drag a suitably contained single AM radio receiver along the bottom of a waterway. The radio is shielded by the water from receiving signals of any significant strength directly from a transmitter. As before, a wire or conductor which has portions above and below water will receive radio signals along its exposed position and retransmit radio signals into the water, which signals affect the radio receiver whenever it passes in close proximity to the wire or conductor. The receiver, which utilizes a horizontal loop antenna, detects the signals and, in the case where amplitude modulation of a carrier exists, converts them to an audio frequency modulation signal for operation of a detection indicator or sounder. In the case where no amplitude modulation exists, a D.C. voltage level corresponding to strength of the carrier signal is used to operate an indicator such as a meter.
Such a single receiver system has been found to be inexpensive and workable, but there are times when interference of various sorts reduce the signal to noise ratio below a level at which detection of a wire can be relied upon. Detection by the aforedescribed loop antenna, amplitude technique is based on inductive coupling between the antenna and the radiating wire and provides an increase in audio signal amplitude due to proximity to the radiation into the water from the submerged portion of the wire as compared to the negligible reception of radio waves directly from the transmitter which arrives at the underwater location of the receiver in a greatly attenuated state. The rate of increase and decrease of the detected signal amplitude is characterized by an extremely sharp peak as the leading edge of the coil passes the wire and another extremely sharp peak as the trailing edge of the coil passes the wire. These closely spaced peaks are so short in duration, that even short lapses in monitoring or short occurrences of noise interference can result in missing a wire detection. Accordingly, with a detector being towed at speeds which can give hope of covering or sweeping a worthwhile area in a given time, it is difficult to provide assurance of wire locations.